Active material for battery, non-aqueous electrolyte battery and battery pack

ABSTRACT

According to one embodiment, a non-aqueous electrolyte battery includes an outer package, a positive electrode housed in the outer package, a negative electrode housed with a space from the positive electrode in the outer package and including an active material, and a non-aqueous electrolyte filled in the outer package. The active material includes a lithium-titanium composite oxide particle, and a coating layer formed on at least a part of the surface of the particle and including at least one metal selected from the group consisting of Mg, Ca, Sr, Ba, Zr, Fe, Nb, Co, Ni, Cu and Si, an oxide of at least one metal selected from the group or an alloy containing at least one metal selected from the group.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a Continuation Application of PCT Application No.PCT/JP2009/053310, filed Feb. 18, 2009, which was published under PCTArticle 21(2) in English.

This application is based upon and claims the benefit of priority fromJapanese Patent Applications No. 2008-064241, filed Mar. 13, 2008; andNo. 2009-006802, filed Jan. 15, 2009; the entire contents of both ofwhich are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to an active material fora battery, a non-aqueous electrolyte battery and a battery pack.

BACKGROUND

Earnest researches and developments have been progressed regardingnon-aqueous electrolyte batteries which charge and discharge electricityby transfer of lithium ions from the negative electrode to the positiveelectrode and vice versa as high-energy density batteries.

These non-aqueous electrolyte batteries are expected to have variouscharacteristics according to their use. They are prospectively used, forexample, at a discharge rate of about 3 C for a power source of adigital camera and at a discharge rate of about 10 C or higher forautomobiles such as hybrid electric cars. Therefore, the non-aqueouselectrolyte batteries used in these fields are desired to havelarge-current characteristics.

Non-aqueous electrolyte batteries obtained using a lithium-transitionmetal composite oxide as the positive electrode active material and acarbonaceous material as the negative electrode active material arecurrently produced on a commercial basis. In these lithium-transitionmetal composite oxides, Co, Mn and Ni are generally used as thetransition metal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a flat type non-aqueous electrolytebattery according to an embodiment.

FIG. 2 is an enlarged sectional view of the part A in FIG. 1.

FIG. 3 is an exploded perspective view showing a battery pack accordingto an embodiment.

FIG. 4 is a block view of a battery pack shown in FIG. 3.

FIG. 5 is an XPS chart of a negative electrode taken out from anon-aqueous electrolyte battery of Example 11 before an evaluation test.

FIG. 6 is an XPS chart of a negative electrode taken out from thenon-aqueous electrolyte battery of Example 11 after the evaluation test.

FIG. 7 is an XPS chart of a negative electrode taken out from anon-aqueous electrolyte battery of Comparative Example 1 before anevaluation test.

FIG. 8 is an XPS chart of a negative electrode taken out from thenon-aqueous electrolyte battery of Comparative Example 1 after theevaluation test.

DETAILED DESCRIPTION

In general, according to one embodiment, an active material for abattery includes a lithium-titanium composite oxide particle, and acoating layer formed on at least a part of the surface of the particleand including at least one metal selected from the group consisting ofMg, Ca, Sr, Ba, Zr, Fe, Nb, Co, Ni, Cu and Si, an oxide of at least onemetal selected from the group or an alloy containing at least one metalselected from the group. Of the group of metals, at least one metalselected from the group consisting of Mg, Fe, Ni and Co is preferable.

A lithium-titanium composite oxide charges lithium at a potential higherthan 1 V (vs.Li/Li⁺). In the case of an active material (e.g., graphiteand a lithium metal) which charges lithium at a potential lower than 1 V(vs.Li/Li⁺), a thick coating film is formed on its surface by thedecomposition of a non-aqueous electrolyte (e.g., a non-aqueouselectrolyte solution) in the first charge operation. The formation ofthe coating film afterward suppresses the decomposition of thenon-aqueous electrolyte solution. However, in the case of alithium-titanium composite oxide which charges lithium at a potentialhigher than 1 V (vs.Li/Li⁺), the decomposition reaction of thenon-aqueous electrolyte solution is so small that a stable coating filmis scarcely formed. As a result, the decomposition reaction of thenon-aqueous electrolyte solution afterward proceeds continuously. Such aphenomenon occurs remarkably in the case of active materials, forexample, Li₄Ti₅O₁₂ having a spinel structure, Li₂Ti₃O₇ having aramsdellite structure, TiO₂ having an anatase structure or TiO₂ having arutile structure, which charge lithium at a potential higher than 1 V(vs.Li/Li⁺).

The active material for a battery according to the embodiment comprisesa lithium-titanium composite oxide particle, and a coating layer formedon at least a part of the surface of the particle and including at leastone metal selected from the group consisting of Mg, Ca, Sr, Ba, Zr, Fe,Nb, Co, Ni, Cu and Si, an oxide of at least one metal selected from thegroup or an alloy containing at least one metal selected from the group,it makes possible to efficiently suppress the decomposition of thenon-aqueous electrolyte solution on the surface of the lithium-titaniumcomposite oxide particle used as the active material. Therefore, thegeneration of gas associated with the decomposition of the non-aqueouselectrolyte solution can be suppressed.

Such an effect is significantly produced when the active material for abattery is used as the negative electrode material and an activematerial (e.g., a lithium-manganese composite oxide) containing Mn isused as the positive electrode active material. That is, it is knownthat when the positive electrode active material containing Mn is used,Mn is eluted in a non-aqueous electrolyte solution. The eluted Mn ionsact on the negative electrode to promote the generation of gas. However,if the negative electrode active material according to the embodiment isused, the influence of these Mn ions can be decreased.

The coating layer including metal, metal oxide or alloy forms the aboveeffect solely on a part of the surface of the lithium-titanium compositeoxide particle. The metal, metal oxide or alloy is desirably formed onthe surface of the lithium-titanium composite oxide particle in an arearatio of 30% or more, more preferably 50% or more and most preferably100% (entire surface).

Examples of the oxide of a metal include an oxide of at least one metalselected from the group of the above metals. These oxides may containunavoidable impurities. An oxide of at least one metal selected from thegroup consisting of Mg, Fe, Ni and Co is preferable.

Examples of the alloy include alloys of two or more metals selected fromthe above group of metals. These alloys may contain unavoidableimpurities. An alloy of two or more metals selected from the groupconsisting of Mg, Fe, Ni and Co is preferable.

The thickness of the coating layer is preferably 1 to 100 nm. When thecoating layer is formed on the entire surface of a lithium-titaniumcomposite oxide particle, the thickness of the coating layer ispreferably defined in this range. When the thickness of the coatinglayer is defined to fall in the range of 1 to 100 nm, the decompositionof the non-aqueous electrolyte solution on the surface of the activematerial can be efficiently suppressed. Also, the definition of thethickness of the coating layer ensures that the lithium-titaniumcomposite oxide particle can keep the same lithium ion charge/dischargeability as lithium-titanium composite oxide particle on which thecoating layer is not formed. Therefore, the active material according tothe present invention can exhibit high energy density and large-currentcharacteristics.

The coating layer of the active material can be formed by coatingmethods such as the CVD method or sputtering method, wet coating methodssuch as the sol gel method or electroless plating or mixing/millingcombined methods such as the ball mill method or jet mill method.

In order to form the coating layer made of the above metal on thesurface of the lithium-titanium composite oxide particle, for example,the electroless plating method can be employed. In the forming thecoating layer including the above metal, a method is effective followingsteps: preparing a negative electrode containing the lithium-titaniumcomposite oxide particle and a non-aqueous electrolyte solution which isdissolved a metal ion containing at least one element selected from theabove group of elements, and precipitating a metal on the surface of thenegative electrode layer by the first charge operation using thesenegative electrode and non-aqueous electrolyte solution, thereby forminga coating layer made of the metal on the surface of the lithium-titaniumcomposite oxide particle. As such a metal ion source, for example, metalsalts such as Ni(BF₄)₂, Co(BF₄)₂ and Fe(BF₄)₂ can be used.

It is preferable to adopt the following methods to form the coatinglayer made of the above metal oxide on the surface of thelithium-titanium composite oxide particle. The lithium-titaniumcomposite oxide particle is introduced into a solution containing onemetal selected from the above group of metals, followed by stirring anddrying, and the obtained particle is sintered at 200 to 800° C. forseveral minutes to several hours to produce an oxide layer of a metalsuch as Mg or Ca on the surface of the lithium-titanium composite oxideparticle, thereby forming the coating layer made of the above metaloxide on the surface of the lithium-titanium composite oxide particle.The above solution may be prepared by dissolving a hydroxide orcarbonate containing at least one metal selected from the above metalgroup in a solvent such as water or ethanol. According to this method, acoating layer made of a metal oxide can be uniformly on the surface of alithium-titanium composite oxide particle having any form. Also, theabove method can improve the adhesion between the coating layer and thelithium-titanium composite oxide particle. Therefore, even ifcharge-discharge operations are repeated for a long period of time, theeffect of limiting the generation of gas along with the decomposition ofthe non-aqueous electrolyte solution can be produced stably.

In the case of the above negative electrode active material, the peaksderived from Co, Fe, Ni or Mg are detected when the negative electrodeactive material is subjected to surface-analysis using X-rayphotoelectron spectroscopy (XPS). The peak derived from the 2p_(3/2)binding energy of Co is detected in the range of 775 to 780 eV, the peakderived from the 2p_(3/2) binding energy of Fe is detected in the rangeof 704 to 709 eV, the peak derived from the 2p_(3/2) binding energy ofNi is detected in the range of 850 to 854 eV and the peak derived fromthe 2p_(3/2) binding energy of Mg is detected in the range of 48 to 50eV. The existence of the peak derived from each element in the aboverange shows that each element exists in a metal (alloy) state in thecoating layer on the surface of the lithium-titanium composite oxideparticle.

The amount each of Co, Fe, Ni and Mg detected by XPS component analysisis preferably 0.1 to 1.0 atomic %. If the amount of each element to bedetected is less than 0.1 atomic %, there is a fear that the effect oflimiting the generation of gas is impaired. If the amount of eachelement to be detected exceeds 1.0 atomic %, on the other hand, thecoating layer itself on the surface of the lithium-titanium compositeoxide particle constitutes a resistance component and there is thereforea fear that the large-current performance is deteriorated.

Examples of the lithium-titanium composite oxide include titanium-basedoxides such as TiO₂, lithium-titanium oxides having, for example, aspinel structure or ramsdellite structure and lithium-titanium compositeoxides obtained by substituting a hetero element for a part of thestructural element. Examples of the lithium-titanium oxide having aspinel structure include Li_(4+x)Ti₅O₁₂ (0≦x≦3) or compounds obtained bysubstituting a part thereof with a heteroatom. Examples of thelithium-titanium composite oxide having a ramsdellite structure includeLi_(2+y)Ti₃O₇ (0≦y≦3) or compounds obtained by substituting a partthereof with a heteroatom. Examples of the titanium-based oxide includetitanium-containing metal composite oxides containing Ti and at leastone element selected from the group consisting of P, V, Sn, Cu, Ni, Feand Co (e.g., TiO₂—P₂O₅, TiO₂—V₂O₅, TiO₂—P₂O₅—SnO₂ or TiO₂—P₂O₅-MeO (Meis at least one element selected from the group consisting of Cu, Ni, Feand Co) besides TiO₂.

The titanium-containing metal composite oxide preferably has amicrostructure in which a crystal phase and an amorphous phase coexistor an amorphous phase singly exists. A titanium-containing metalcomposite oxide having such a microstructure enables a substantiallyhigh capacity to be drawn even in a high-rate charge-discharge operationand can also be remarkably improved in cycle performance.

The lithium-titanium composite oxide is preferably a lithium titanatehaving a spinel structure from the viewpoint of cycle life. Among theselithium titanates, Li_(4+x)Ti₅O₁₂ (0≦x≦3) having a spinel structure ispreferable because it is superior in initial charge-discharge efficiencyand has a high effect on an improvement in cycle characteristics.

The molar ratio of oxygen in the lithium-titanium composite oxide isshown formally as 12 in the case of Li_(4+x)Ti₅O₁₂ (0≦x≦3) having aspinel structure and as 7 in the case of Li_(2+y)Ti₃O₇ (0≦y≦3). Thesemolar ratios may be varied by the influence of oxygen non-stoichiometry.

The lithium-titanium composite oxide particles preferably have anaverage particle diameter of 1 μm or less and a specific surface area of5 to 50 m²/g. The specific surface area is measured by the BET methodusing N₂ adsorption. Lithium-titanium composite oxide particles havingsuch an average particle diameter and a specific surface area can beimproved in utilization factor, which enables a substantially highcapacity to be drawn even in a high charge-discharge operation. Here,the BET specific surface area by N₂ gas adsorption may be measured byusing a Micromeritex ASAP-2010 produced by Shimadzu Corporation and N₂as the adsorbing gas.

The active material for a battery according to this embodiment may beused not only for a negative electrode but also for a positiveelectrode. The active material for a battery can efficiently suppressthe decomposition of a non-aqueous electrolyte (e.g., a non-aqueouselectrolyte solution) which occurs on the surface of a lithium compositeoxide, which is the active material, even if it is applied to any ofthese electrodes. In other words, the effect of limiting thedecomposition of the non-aqueous electrolyte solution is due to thelithium working potential (1 to 2 V vs. Li/Li⁺) of the lithium-titaniumcomposite oxide and therefore, the effects on both electrodes are notdifferent. Therefore, the active material for a battery according tothis embodiment may be used for both the positive electrode and thenegative electrode and the same effect can be obtained.

When the active material for a battery according to this embodiment isused for the positive electrode, a metal lithium, a lithium alloy or acarbon-based material such as graphite and coke may be used for anactive material of the negative electrode, which is the counterelectrode.

Next, the non-aqueous electrolyte battery according to this embodimentwill be described in detail.

In general, according to another embodiment, a non-aqueous electrolytebattery includes: an outer package; a positive electrode housed in theouter package; a negative electrode housed with a space from thepositive electrode, for example, with a separator being interposedbetween these electrodes, in the outer package and including an activematerial; and a non-aqueous electrolyte filled in the outer package,wherein the active material comprises a lithium-titanium composite oxideparticle, and a coating layer formed on at least a part of the surfaceof the particle and including at least one metal selected from the groupconsisting of Mg, Ca, Sr, Ba, Zr, Fe, Nb, Co, Ni, Cu and Si, an oxide ofat least one metal selected from the group or an alloy containing atleast one metal selected from the group.

The above outer package, negative electrode, non-aqueous electrolyte,positive electrode and separator will be described in detail.

1) Outer Package

As the package, a container made of a laminate film having 0.5 mm orless in thickness or a metal film having 1.0 mm or less in thickness maybe used. The thickness of the metal container is more preferably 0.5 mmor less.

Examples of the form of the outer package include a flat shape (thinshape), angular shape, cylindrical shape, coin shape and button shape.Given as examples of the outer package are outer packages forsmall-sized batteries to be mounted on, for example, mobile electronicdevices and outer packages for large-sized batteries to be mounted ontwo-wheeled or four-wheeled vehicles according to the dimensions of abattery.

As the laminate film, a multilayer film prepared by interposing a metallayer between resin layers may be used. The metal layer is preferablymade of an aluminum foil or an aluminum alloy foil to develop alightweight battery. As the resin layer, a high-molecular material suchas a polypropylene (PP), polyethylene (PE), nylon or polyethyleneterephthalate (PET) may be used. A laminate film can be molded into theshape of the outer package by carrying out thermal fusion to seal up themolded material.

The metal container is made of aluminum or an aluminum alloy. As thealuminum alloy, alloys containing elements such as magnesium, zinc andsilicon are preferable. When transition metals such as iron, copper,nickel and chromium are contained in the alloy, the amount of thesetransition metals is preferably designed to be 100 ppm or less.

2) Negative Electrode

The negative electrode comprises a current collector and a negativeelectrode layer which is formed on one or both surfaces of the currentcollector and contains an active material, a conductive agent and abinder.

As the active material, the above active material for a battery, whichcomprises a lithium-titanium composite oxide particle, and a coatinglayer formed on at least a part of the surface of the particle andincluding at least one metal selected from the group consisting of Mg,Ca, Sr, Ba, Zr, Fe, Nb, Co, Ni, Cu and Si, an oxide of at least onemetal selected from the group or an alloy containing at least one metalselected from the group can be used.

As the conductive agent, a carbon material may be used. Examples of thecarbon material include acetylene black, carbon black, cokes, carbonfibers or graphite, and also include a metal powder such as an aluminumpowder or conductive ceramics such as TiO. Among these materials, cokesand graphite which are heat-treated at 800 to 2000° C. and have anaverage particle diameter of 10 μm or less and carbon fibers having anaverage particle diameter of 1 μm or less are preferable. The BETspecific surface area of the carbon material which is measured by N₂adsorption is preferably 10 m²/g or more.

Examples of the binder include a polytetrafluoroethylene (PTFE),polyvinylidene fluoride (PVdF), fluorine-based rubber, styrene-butadienerubber or core-shell binder.

The proportions of the active material, the conductive agent and thebinder are preferably as follows: the amount of the active material is70% to 96% by weight, the amount of the conductive agent is 2% to 28% byweight and the amount of the binder is 2% to 28% by weight. When theamount of the conductive agent is less than 2% by weight, there is afear that the current-collecting performance of the negative electrodelayer is deteriorated and the large-current characteristics of thenon-aqueous electrolyte secondary battery are therefore deteriorated.When the amount of the binder is less than 2% by weight, there is a fearas to a deterioration in binding characteristics between the negativeelectrode layer and the negative electrode current collector and hence adeterioration in cycle characteristics. The amounts of the conductiveagent and binder are respectively preferably 28% by weight or less fromthe viewpoint of increasing the capacity of the battery.

The porosity of the negative electrode layer is preferably 20 to 50% byvolume. The negative electrode provided with the negative electrodelayer having such a porosity is highly densified and is superior inaffinity to the non-aqueous electrolyte. The porosity is furtherpreferably 25 to 40% by volume.

The current collector is preferably made of an aluminum foil or aluminumalloy foil. The current collector preferably has an average crystalparticle diameter of 50 μm or less. This can outstandingly improve thestrength of the current collector and therefore, the negative electrodecan be highly densified by pressing with a large pressing force,enabling the battery capacity to be increased. Also, because thedissolution and corrosive deterioration of the current collector in anovercharge cycle under a high-temperature environment (40° C. or higher)can be prevented, a rise in negative electrode impedance can besuppressed. Moreover, the battery can also be improved in outputcharacteristics, rapid charging characteristics and charge-dischargecycle characteristics. The average crystal particle diameter is morepreferably 30 μm or less and even more preferably 5 μm or less.

The average crystal particle diameter is determined in the followingmanner. The tissue of the surface of the current collector is observedby an optical microscope to find the number n of crystal particlesexisting in an area of 1 mm×1 mm. The average crystal particle area S isdetermined by using the obtained n from the equation S=1×10⁶/n (μm²).From the obtained value of S, the average crystal particle diameter d(μm) is calculated according to the following equation (1).

d=2(S/π)^(1/2)  (1)

An aluminum foil or aluminum alloy foil of which the above averagecrystal particle diameter is in a range of 50 μm or less is complexlyaffected by a plurality of factors such as material textures,impurities, processing conditions, heat treating hysteresis andannealing conditions and the above crystal particle diameter is adjustedby appropriate combination of the above factors in the productionprocess.

The thickness of the aluminum foil or aluminum alloy foil is preferably20 μm or less and more preferably 15 μm or less. The purity of thealuminum foil is preferably 99% by weight or higher. As the aluminumalloy, alloys containing an element such as magnesium, zinc or siliconare preferable. On the other hand, it is preferable that transitionmetals such as iron, copper, nickel or chromium be contained in anamount of 1% by weight or lower in the aluminum alloy.

The negative electrode is produced, for example, by suspending theactive material, conductive agent and binder in a commonly-used solventto prepare a slurry, which is then applied to a current collector,followed by drying to form a negative electrode layer, which is thenpressed. Other than the above method, the active material, conductiveagent and binder may be formed into a pellet, which is used as anegative electrode layer.

In the case of such a negative electrode, the peaks derived from Co, Fe,Ni or Mg are detected when the negative electrode is subjected tosurface-analysis using X-ray photoelectron spectroscopy (XPS). That is,the peak derived from the 2p_(3/2) binding energy of Co is detected inthe range of 775 to 780 eV, the peak derived from the 2p_(3/2) bindingenergy of Fe is detected in the range of 704 to 709 eV, the peak derivedfrom the 2p_(3/2) binding energy of Ni is detected in the range of 850to 854 eV and the peak derived from the 2p_(3/2) binding energy of Mg isdetected in the range of 48 to 50 eV. The existence of the peak derivedfrom each element in the above range shows that each element exists in ametal (alloy) state in the coating layer on the surface of thelithium-titanium composite oxide particle in the negative electrode(negative electrode layer).

The amount of Co, Fe, Ni or Mg detected by XPS component analysis ispreferably 0.1 to 1.0 atomic %. If the amount of each element to bedetected is less than 0.1 atomic %, there is a fear that the effect oflimiting the generation of gas is impaired. If the amount of eachelement to be detected exceeds 1.0 atomic %, on the other hand, thecoating layer itself on the surface of the lithium-titanium compositeoxide particle constitutes a resistance component and there is thereforea fear that the large-current performance is deteriorated.

3) Non-Aqueous Electrolyte

Examples of the non-aqueous electrolyte include a liquid non-aqueouselectrolyte and a liquid non-aqueous electrolyte obtained by making acomposite of a liquid electrolyte and a high-molecular material.

The liquid non-aqueous electrolyte is prepared by dissolving anelectrolyte into an organic solvent in a concentration of 0.5 to 2.5mol/L.

Examples of the electrolyte include lithium salts such as lithiumperchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithiumtetrafluorophosphate (LiPF₄), lithium arsenichexafluoride (LiAsF₆),lithium trifluoromethanesulfonate (LiCF₃SO₃) andbistrifluoromethylsulfonylimide lithium (LiN(CF₃SO₂)₂) or mixtures ofthese lithium salts. Those which are scarcely oxidized even at a highpotential are preferable, and LiPF₆ is most preferable.

The organic solvent may be used a single solvent or mixtures of solventsselected from cyclic carbonates such as propylene carbonate (PC),ethylene carbonate (EC) and vinylene carbonates; chain carbonates suchas diethyl carbonate (DEC), dimethyl carbonate (DMC) and methylethylcarbonate (MEC); cyclic ethers such as tetrahydrofuran (THF),2-methyltetrahydrofuran (2MeTHF) and dioxolan (DOX); chain ethers suchas dimethoxyethane (DME) and diethoxyethane (DEE); γ-butyrolactone(GBL), acetonitrile (AN) and sulfolane (SL).

Examples of the high-molecular material include a polyvinylidenefluoride (PVdF), polyacrylonitrile (PAN) and polyethylene oxide (PEO).

The organic solvent is preferably a mixed solvent obtained by blendingtwo or more solvents selected from the group consisting of propylenecarbonate (PC), ethylene carbonate (EC) and γ-butyrolactone (GBL). Theorganic solvent is further preferably γ-butyrolactone (GBL). This reasonis as follows.

The lithium-titanium composite oxide phase, which is a chief material ofthe negative electrode active material, charges and discharges lithiumions at a potential range of 1 to 2 V (vs. Li/Li⁺). However, thereduction and decomposition of the non-aqueous electrolyte are scarcelycaused in this potential range, so that a coating film, which is areduction product, is formed with difficulty on the surface of thelithium-titanium composite oxide. Therefore, when the lithium-titaniumcomposite oxide exists in the situation where lithium is chargedtherein, that is, in fully charged state, lithium ions charged in thelithium-titanium composite oxide gradually diffuse into the electrolytesolution, causing the so-called self discharge. Such self-dischargearises significantly when the battery is put into a high-temperaturestorage environment.

Of these organic solvents, γ-butyrolactone is more reducible than achain carbonate or cyclic carbonate. These organic compounds describedin the order of reducibility are as follows: γ-butyrolactone>>>ethylenecarbonate>propylene carbonate>>dimethyl carbonate>methylethylcarbonate>diethyl carbonate. Therefore, a favorable coating film can beformed on the surface of the lithium-titanium composite oxide even inthe working potential range of the lithium-titanium composite oxide byplacing γ-butyrolactone in the electrolyte solution. As a result,self-discharge can be suppressed, thereby making it possible to improvethe high-temperature storage characteristics of the non-aqueouselectrolyte battery.

The mixed solvent obtained by blending two or more solvents selectedfrom the group consisting of propylene carbonate (PC), ethylenecarbonate (EC) and γ-butyrolactone (GBL), and a mixed solvent includingγ-butyrolactone can likewise limit the self-discharge, thereby making itpossible to improve the high-temperature storage characteristics of thenon-aqueous electrolyte battery.

γ-butyrolactone is preferable because a high-quality protective coatingfilm can be formed by blending it in an amount of 40% to 95% by volumebased on the organic solvent.

4) Positive Electrode

The positive electrode comprises a current collector and a positiveelectrode layer which is formed on one or both surfaces of the currentcollector and contains an active material, a conductive agent and abinder.

The current collector is preferably made of an aluminum foil or analuminum alloy foil containing elements such as Mg, Ti, Zn, Mn, Fe, Cuand Si.

The active material may be used, for example, an oxide or a polymer.

The oxides may be used, for example, manganese oxide (MnO₂), iron oxide,copper oxide and nickel oxide in which lithium is charged,lithium-manganese composite oxide (e.g., Li_(x)Mn₂O₄ or Li_(x)MnO₂),lithium-nickel composite oxide (e.g., L_(x)NiO₂), lithium-cobaltcomposite oxide (e.g., Li_(x)CoO₂), lithium-nickel-cobalt compositeoxide (e.g., LiNi_(1−y)CO_(y)O₂), lithium-manganese-cobalt compositeoxide (e.g., Li_(x)Mn_(y)CO_(1−y)O₂), spinel typelithium-manganese-nickel composite oxide e.g., Li_(x)Mn_(2−y)Ni_(y)O₄),lithium-phosphorous oxide having an olivine structure (e.g.,Li_(x)FePO₄, Li_(x)Fe_(1−y)Mn_(y)PO₄ and Li_(x)CoPO₄), iron sulfate(Fe₂(SO₄)₃) or vanadium oxide (e.g., V₂O₅). Here, x and y are preferably0<x≦1 and 0<y≦1, respectively.

The polymer may be used, for example, conductive polymer materials suchas a polyaniline and polypyrrole and disulfide-based polymers may beused. Also, sulfur (S), fluorocarbon and the like.

Preferable examples of the active material include lithium-manganesecomposite oxide (Li_(x)Mn₂O₄), lithium-nickel composite oxide(Li_(x)NiO₂), lithium-cobalt composite oxide (Li_(x)CoO₂),lithium-nickel-cobalt composite oxide (Li_(x)Ni_(1−y)CO_(y)O₂), spineltype lithium-manganese-nickel composite oxide (Li_(x)Mn_(2−y)Ni_(y)O₄),lithium-manganese-cobalt composite oxide (Li_(x)Mn_(y)CO_(1−y)O₂) andlithium ironphosphate (Li_(x)FePO₄) which each provide a high positiveelectrode voltage. Here, x and y preferably satisfy the followingrelations: 0<x≦1 and 0<y≦1.

The active material is more preferably a lithium-cobalt composite oxideor lithium-manganese composite oxide. Because these materials have highion conductivity, there are difficulties in carrying out the process inwhich the diffusion of lithium ions in the active material is therate-determining step when these materials are combined with thenegative electrode material of the embodiment. Therefore, these activematerials are superior in adaptability to the lithium-titanium compositeoxide in the negative electrode active material used in this embodiment.

In the non-aqueous electrolyte battery according to the embodiment, ahigher effect can be obtained by combining a positive electrodeincluding a Mn-containing active material, which gives rise to theelution of Mn as mentioned above, with the negative electrode includingan active material which comprises a lithium-titanium composite oxideparticle, and a coating layer formed on at least a part of the surfaceof the particle and including at least one metal selected from the groupconsisting of Mg, Ca, Sr, Ba, Zr, Fe, Nb, Co, Ni, Cu and Si, an oxide ofat least one metal selected from the group or an alloy containing atleast one metal selected from the group. An even higher effect can beobtained when a manganese-containing lithium-transition metal compositeoxide having a spinel structure is used as the positive electrode activematerial. Examples of the manganese-containing lithium-transition metalcomposite oxide having a spinel structure include Li_(x)Mn_(2−y)M_(y)O₄(0≦x≦1.2 and 0≦y≦1, M is an element other than Mn). As the M element, Coand Al may be used to produce an effect of decreasing the elution amountof Mn.

The primary particle diameter of the active material is designed to be100 nm to 1 μm, which is preferable as regards handling in industrialproduction and to progress the diffusion of lithium ions in a solidsmoothly.

The specific surface area of the active material is designed to be 0.1to 10 m²/g, which is preferable because the charge/discharge site oflithium ions can be sufficiently secured, handling in industrialproduction is made easy and more favorable charge-discharge cyclecharacteristics can be secured.

The conductive agent may be used, for example, a carbonaceous materialsuch as acetylene black, carbon black or graphite. These conductiveagents can improve the current-collecting ability and can suppress thecontact resistance with the current collector.

The binder may be used a polytetrafluoroethylene (PTFE), polyvinylidenefluoride (PVdF) or fluorine-based rubber.

With regard to each ratio of the active material, conductive agent andbinder to be compounded, it is preferable that the active material is80% to 95% by weigh, the conductive agent is 3% to 18% by weight and thebinder is 2% by to 17% by weight. The conductive agent produces the higheffect when it is compounded in an amount of 3% by weight or more. Also,the conductive agent can reduce the decomposition of the non-aqueouselectrolyte on the surface thereof under a high-temperature storagecondition when it is compounded in an amount of 10% by weight or less.The binder succeeds in obtaining a satisfactory electrode strength whenit is compounded in an amount of 2% by weight or more. Also, when thebinder is compounded in an amount of 10% by weight or less, it reducesthe amount of the insulating material in the electrode to thereby reducethe internal resistance.

The positive electrode is produced by suspending the active material,the conductive agent and the binder in an appropriate solvent to preparea slurry, which is then applied to a positive electrode currentcollector, followed by drying to produce a positive electrode layer,followed by pressing. Other than the above, the active material,conductive agent and binder may be formed into a pellet, which is usedas the positive electrode layer.

5) Separator

Examples of the separator include porous films or synthetic resinnonwoven fabrics containing a polyethylene, polypropylene, cellulose orpolyvinylidene fluoride (PVdF). Of these materials, since a porous filmmade of a polyethylene or polypropylene melts at a fixed temperature tobe able to cut off current, it is favorable from the viewpoint ofimproving safety.

Next, the non-aqueous electrolyte battery (e.g., a flat type non-aqueouselectrolyte battery provided with an outer package constituted of alaminate film) according to this embodiment will be described in detailwith reference to FIGS. 1 and 2. FIG. 1 is a sectional view of a thintype non-aqueous electrolyte battery and FIG. 2 is an enlarged sectionalview of the part A in FIG. 1. Each of these drawings is a typical viewfor describing the invention and for promoting the understanding of theinvention. Though there is a case where shapes, dimensions and ratios ofthe battery described in these drawings differ from those of the actualequipment, the designs of these parts may be appropriately changed inconsideration of the following invention and known technologies.

A flattened wound electrode group 1 is housed in a bag-shaped outerpackage 2 made of a laminate film obtained by interposing an aluminumfoil between two resin layers. The flattened wound electrode group 1 isconstructed by spirally wounding and press-molding a laminate comprisingmentioning from outside, a negative electrode 3, a separator 4, apositive electrode 5 and a separator 4. As shown in FIG. 2, the negativeelectrode 3 constituting the outermost husk has a structure in which anegative electrode layer 3 b is formed on one surface of a currentcollector 3 a, the negative electrode layer 3 b containing an activematerial which comprises a lithium-titanium composite oxide particle,and a coating layer formed on at least a part of the surface of theparticle and including at least one metal selected from the groupconsisting of Mg, Ca, Sr, Ba, Zr, Fe, Nb, Co, Ni, Cu and Si, an oxide ofat least one metal selected from the group or an alloy containing atleast one metal selected from the group. Other negative electrodes 3 areconstituted by forming negative electrode layers 3 b on the bothsurfaces of the current collectors 3 a. The positive electrode 5 isstructured by forming positive electrode layers 3 b on the both surfacesof the current collector 5 a.

In the vicinity of the outside peripheral end of the flattened woundelectrode group 1, a negative electrode terminal 6 is connected to thecurrent collector 3 a of the negative electrode 3 in the outermost huskand a positive electrode terminal 7 is connected to the currentcollector 5 a of the positive electrode 5 disposed inside of thenegative electrode. The negative electrode terminal 6 and positiveelectrode terminal 7 are protruded out of the bag-shaped outer package 2from an opening formed therein. For example, a liquid non-aqueouselectrolyte is injected from an opening of the bag-shaped outer package2. The opening of the bag-shaped outer package 2 is heat-sealed withholding the negative electrode terminal 6 and positive electrodeterminal 7 to thereby completely seal the coiled electrode group 1 andthe liquid non-aqueous electrolyte.

The negative electrode terminal may be used, for example, a materialhaving electrical stability and conductivity within a potential of 1.0to 3.0 V with respect to a lithium ion metal. Examples of the materialof the negative electrode terminal include aluminum and aluminum alloyscontaining elements such as Mg, Ti, Zn, Mn, Fe, Cu and Si. The negativeelectrode terminal is preferably made of the same material as thecurrent collector in order to reduce the contact resistance with thecurrent collector.

The positive electrode terminal may be used, for example, a materialhaving electrical stability and conductivity within a potential of 3.0to 4.25 V with respect to a lithium ion metal. Examples of the materialof the positive electrode terminal include aluminum and aluminum alloyscontaining elements such as Mg, Ti, Zn, Mn, Fe, Cu and Si. The positiveelectrode terminal is preferably made of the same material as thecurrent collector in order to reduce the contact resistance with thecurrent collector.

According to the non-aqueous electrolyte battery of such embodiment, itis provided with the negative electrode including the active materialwhich comprises a lithium-titanium composite oxide particle, and acoating layer formed on at least a part of the surface of the particleand including at least one metal selected from the group consisting ofMg, Ca, Sr, Ba, Zr, Fe, Nb, Co, Ni, Cu and Si, an oxide of at least onemetal selected from the group or an alloy containing at least one metalselected from the group, whereby the decomposition reaction between thenon-aqueous electrolyte (e.g., a non-aqueous electrolyte solution) andthe surface of the lithium-titanium composite oxide particles can beefficiently suppressed. As a result, the generation of gas on thesurface of the negative electrode can be suppressed and a non-aqueouselectrolyte battery reduced in the swelling of the outer package can beprovided. Such an effect can be obtained more significantly when thisnegative electrode is combined with a positive electrode containing anMn-containing positive electrode active material.

In general, according to another embodiment, a battery pack includes aplurality of the above non-aqueous electrolyte batteries, for exampleunit cells, which are electrically connected each other in series, inparallel, or both in series and in parallel.

The non-aqueous electrolyte battery according to this embodiment ispreferably used as each of the unit cells forming a battery module. Theobtained battery pack has excellent cycle characteristics.

In the decomposition reaction of the non-aqueous electrolyte whicharises on the surface of the negative electrode and on the surface ofthe lithium-titanium composite oxide used as the active material, theamount of reaction varies corresponding to the environmental temperatureand increases with increase in environmental temperature as mentionedabove. The battery module is constituted, for example, by combining aplurality of unit cells. Therefore, the heat of a unit cell disposed atthe outermost position tends to be released and therefore thetemperature of the unit cell tends to be decreased. On the other hand,the heat of a unit cell disposed inside tends to be scarcely releasedand therefore the temperature of the unit cell tends to be scarcelydecreased. In other words, the temperature in the battery module variesdepending on the position, and therefore the temperatures of the unitcells tend to be different from each other. As a result, the amount ofthe non-aqueous electrolyte solution to be decomposed is larger in unitcells disposed inside than in unit cells disposed outside. Thedecomposition of the non-aqueous electrolyte solution on the surface ofthe negative electrode reduces the charge-discharge efficiency of thenegative electrode, which destroys the balance in capacity between thepositive electrode and the negative electrode. Such adverse capacitybalance causes a part of the batteries to be put in an overcharge state,which shortens the cycle life of the battery module.

In the case of constituting a battery module by using the non-aqueouselectrolyte battery according to this embodiment as unit cells, theamount of the decomposition of the non-aqueous electrolyte solution inall of these unit cells can be reduced, which makes the battery moduleresistant to the influence of temperature unevenness, so that the cyclelife of the battery module can be prolonged.

One example of such a battery pack will be explained in detail withreference to FIGS. 3 and 4. The single cell can be employed the flattingtype battery as shown in FIG. 1.

A plurality of single cells 21, each formed of the flatting typenon-aqueous electrolyte battery shown in FIG. 1, are laminated in such amanner that the negative electrode terminal 6 and the positive electrodeterminal 7, both being externally led out, are arrayed to extend in thesame direction and that they are clamped together by means of anadhesive tape 22, thereby creating a combined battery 23. These singlecells 21 are electrically connected with each other in series as shownin FIG. 4.

A printed wiring board 24 is disposed to face the side wall of each ofthe single cells 21 where the negative electrode terminal 6 and thepositive electrode terminal 7 are externally led out. On this printedwiring board 24 are mounted a thermistor 25, a protection circuit 26,and a terminal 27 for electrically connecting the printed wiring board24 with external instruments. It should be noted that in order toprevent unwanted electric connection with the wirings of the combinedbattery 23, an insulating plate (not shown) is attached to the surfaceof the printed wiring board 24 that faces the combined battery 23.

A lead 28 for the positive electrode is electrically connected, throughone end thereof, with the positive electrode terminal 7 which is locatedat the lowest layer of the combined battery 23. The other end of thelead 28 is inserted into and electrically connected with a connector 29for the positive terminal of the printed wiring board 24. A lead 30 forthe negative electrode is electrically connected, through one endthereof, with the negative electrode terminal 6 which is located at thehighest layer of the combined battery 23. The other end of the lead 30is inserted into and electrically connected with a connector 31 for thenegative terminal of the printed wiring board 24. These connectors 29and 31 are electrically connected, through the interconnects 32 and 33formed on the printed wiring board 24, with the protection circuit 26.

The thermistor 25 is used for detecting the temperature of single cells21 and the signals thus detected are transmitted to the protectioncircuit 26. This protection circuit 26 is designed to cut off, underprescribed conditions, the wiring 34 a of plus-side and the wiring 34 bof minus-side which are interposed between the protection circuit 26 andthe terminal 27 for electrically connecting the printed wiring board 24with external instruments. The expression of “under prescribedconditions” herein means the conditions where the temperature detectedby the thermistor 25 becomes higher than a predetermined temperature forexample. Further, the expression of “under prescribed conditions” hereinalso means the conditions where the over-charging, over-discharging andover-current of the single cells 21 are detected. The detection of thisover-charging is performed against the single cells 21 individually orentirely. In the case where the single cells 21 are to be detectedindividually, either the voltage of cell may be detected or thepotential of the positive electrode or negative electrode may bedetected. In the latter case, a lithium electrode is inserted, as areference electrode, into individual cells 21. In the case of thebattery pack shown in FIGS. 3 and 4, a wiring 35 is connected with eachof the single cells 21 for detecting the voltage thereof and the signalsdetected are transmitted, through this wiring 35, to the protectioncircuit 26.

On all of the sidewalls of the combined battery 23 excepting onesidewall where the negative electrode terminal 6 and the positiveelectrode terminal 7 are protruded, a protective sheet 36 made of rubberor synthetic resin is disposed, respectively.

The combined battery 23 is housed, together with each of protectivesheet 36 and the printed wiring board 24, in a case 37. Namely, theprotective sheet 36 is disposed on the opposite inner sidewallsconstituting the longer sides of the case 37 and on one inner sidewallconstituting one shorter side of the case 37. On the other sidewallconstituting the other shorter side of the case 37 is disposed theprinted wiring board 24. The combined battery 23 is positioned in aspace which is surrounded by the protective sheet 36 and the printedwiring board 24. A cap 38 is attached to the top of the case 37.

In this case, the battery module 23 may be secured by using aheat-shrinkable tube in place of the adhesive tape 22. In addition, aprotective sheet is disposed on each side surface of the battery moduleand the heat-shrinkable tube is wound around the protective sheets.Then, the heat-shrinkable tube is heat-shrunk to closely bind thebattery module.

In FIGS. 3 and 4, though the unit cells 21 are shown in the embodimentin which they are connected in series, they may be connected in parallelor in a combination of series and parallel to increase the capacity ofthe battery. Also, the produced battery packs may be further connectedin series or in parallel.

Also, the structure of the battery pack is appropriately changedaccording to use. The applications of the battery pack are preferablythose for which cycle characteristics in large-current characteristicsare desired. More specifically, examples of applications include powersources used in, for example, digital cameras and power sources used inautomobiles, for example, hybrid two-wheeled or four-wheeledelectromobiles, two-wheeled or four-wheeled electric cars andpower-assisted bicycles. Applications as power sources for automobilesare preferable.

As mentioned above, a non-aqueous electrolyte battery superior inhigh-temperature characteristics can be obtained by using a non-aqueouselectrolyte containing either a mixture solvent obtained by mixing twoor more solvents selected from the group consisting of propylenecarbonate (PC), ethylene carbonate (EC) and γ-butyrolactone (GBL) orγ-butyrolactone (GBL). A battery pack provided with a battery modulehaving a plurality of these non-aqueous electrolyte batteries ispreferable as a power source for automobiles.

The present invention will be described in more detail by way ofexamples. However, these examples are not intended to limit the scope ofthe present invention.

Example 1 Production of a Positive Electrode

First, 90% by weight of a lithium-manganese oxide (LiMn_(1.9)Al_(0.1)O₄)powder having a spinel type structure which was used as a positiveelectrode active material, 5% by weight of acetylene black used as aconductive agent and 5% by weight of a polyvinylidene fluoride (PVdF)were added in N-methylpyrrolidone (NMP) and these compounds were mixedto prepare a slurry. This slurry was applied to both surfaces of acurrent collector made of an aluminum foil 15 μm in thickness, followedby drying and pressing to produce a positive electrode having anelectrode density of 2.9 g/cm³.

<Production of a Lithium-Titanium Composite Oxide Particle (1)>

First, Li₂CO₃ and anatase type TiO₂ were mixed such that the molar ratioof Li:Ti was 4:5 and the mixture was calcined at 850° C. for 12 hours inair to thereby obtain a spinel type lithium-titanium composite oxideLi₄Ti₅O₁₂ (precursor).

The synthesized Li₄Ti₅O₁₂ (92 g) was poured into a solution obtained bydissolving Mg(OH)₂ (0.175 g) in water, followed by stirring and drying,and then calcined at 400° C. for 3 hours to obtain a granular negativeelectrode active material.

The obtained negative electrode active material was subjected to FIB-TEManalysis and EDX analysis. As a result, it was confirmed that an Mgoxide layer, i.e., a coating layer having 5 to 10 nm in thickness wasformed over the entire surface of Li₄Ti₅O₁₂ particles. Also, it wasfound from the result of XPS that Mg existed in the form of Mg²⁺, and itwas therefore inferred that the Mg oxide was MgO. Also, it was confirmedfrom the result of powder X-ray diffraction method that the Mg oxide wasamorphous.

<Production of a Negative Electrode>

90% by weight of the obtained lithium-titanium composite oxide particleshaving Mg oxide layer as the active material, 5% by weight of coke(d002: 0.3465 nm, average particle diameter: 3 μm) calcined at 1200° C.as a conductive agent and 5% by weight of a polyvinylidene fluoride(PVdF) were added in N-methylpyrrolidone (NMP) and these components weremixed to prepare a slurry. This slurry was applied to both surfaces of acurrent collector made of an aluminum foil 15 μm in thickness and dried,followed by pressing to produce a negative electrode having an electrodedensity of 2.0 g/cm³.

The average particle diameter of the lithium-titanium composite oxideparticles was 0.96 μm. The method for measuring the average particlediameter will be described below.

The average particle diameter was measured in the following manner byusing a laser diffraction type distribution-measuring device (tradename: SALD-3000, produced by Shimadzu Corporation). First, a beaker wascharged with about 0.1 g of a sample, a surfactant and 1 to 2 mL ofdistilled water, which were stirred sufficiently, and then the mixturewas poured into a stirring water bath and the distribution of luminositywas measured 64 times at intervals of 2 seconds to analyze the data ofthe grain size distribution.

<Production of an Electrode Group>

The above-described positive electrode, a sheet of separator made of aporous polyethylene film having a thickness of 25 μm, theabove-described negative electrode, and another sheet of separator madeof the same kind of film as described above are laminated in thementioned order and then spirally wound to form a wound body, which wasthermally press at a temperature of 90° C. to manufacture a flattenedwound electrode group. The obtained electrode group was housed in a packmade of an aluminum laminate film, which was then dried under vacuum at80° C. for 24 hours.

<Preparation of a Liquid Non-Aqueous Electrolyte>

1.5 mol/L of LiBF₄ used as an electrolyte was dissolved in a mixedsolution prepared by blending ethylene carbonate (EC) andγ-butyrolactone (GBL) in a ratio by volume of 1:2 to prepare a liquidnon-aqueous electrolyte.

The liquid non-aqueous electrolyte was poured into the laminate filmpack in which the electrode group was housed. After that, the pack wascompletely sealed by heat sealing to assemble a non-aqueous electrolytesecondary battery which had the structure shown in FIG. 1 and had awidth of 70 mm, a thickness of 6.5 mm and a height of 120 mm.

Comparative Example 1

A non-aqueous electrolyte secondary battery was assembled in the samemanner as in Example 1 except that the precursor (spinel typelithium-titanium composite oxide Li₄Ti₅O₁₂), which was synthesized inExample 1 and was not surface-modified, was used as the negativeelectrode active material.

Examples 2 to 14 and Comparative Examples 2 to 4

Non-aqueous electrolyte secondary batteries were assembled in the samemanner as in Example 1 except that each of the coating layers made ofthe coating materials shown in the following Table 1 is formed on theentire surface of the spinel type lithium-titanium composite oxideparticle. When the coating material is a metal, the electroless platingmethod was used as the coating method. Also, all of these coatingmaterials were amorphous and had a thickness of 5 to 10 nm.

Example 15

A non-aqueous electrolyte secondary battery was assembled in the samemanner as in Example 1 except that the synthesized Li₄Ti₅O₁₂ obtainedExample 1 was used as an active material and 1500 ppm of Fe(BF₄)₂ wasadded to liquid non-aqueous electrolyte.

Example 16

A non-aqueous electrolyte secondary battery was assembled in the samemanner as in Example 1 except that the synthesized Li₄Ti₅O₁₂ obtainedExample 1 was used as an active material and 1500 ppm of Ni(BF₄)₂ wasadded to liquid non-aqueous electrolyte.

Example 17

A non-aqueous electrolyte secondary battery was assembled in the samemanner as in Example 1 except that the synthesized Li₄Ti₅O₁₂ obtainedExample 1 was used as an active material and 1500 ppm of Co(BF₄)₂ wasadded to liquid non-aqueous electrolyte.

Each of batteries obtained in Examples 1 to 17 and Comparative Examples1 to 4 was subjected to a high-temperature storage test conducted at 60°C. for 4 weeks in a 2.55 V-charged state to measure the thicknesses ofthe battery before and after it was stored. From these thicknesses ofthe battery, a change in the thickness of the battery was determinedaccording to the following equation.

Variation in the thickness of the battery (times)=(Thickness of thebattery after stored/Thickness of the battery before stored)

Also, with regard to each battery obtained in Examples 1 to 17 andComparative Examples 1 to 4, the DC resistance of the battery before thebattery was stored was measured to calculate the ratio (times) of theresistance of the battery to the reference resistance, which was that ofthe battery of Comparative Example 1, using the no material coveringnegative electrode active material. These results are shown in Table 1below. In this case, the DC resistance (R) was calculated from thedifference in voltage between 10 C discharge and 1 C discharge. Thebattery was allowed to discharge under 10 C current (C₁) and 1 C current(C₂) for 0.2 seconds to measure the voltages V₁ and V₂ after dischargedrespectively and the DC resistance was calculated from the equation:

R=(V ₂ −V ₁)/(C ₁ −C ₂).

TABLE 1 Change in the Coating material of thickness of Battery spinellithium-titanium the battery resistance composite oxide particles(times) (times) Comparative Non >2 1.00 Example 1 Example 1 Mgoxide(MgO) 1.05 0.95 Example 2 Ca oxide(CaO) 1.07 1.05 Example 3 Sroxide(SrO) 1.08 1.05 Example 4 Ba oxide(BaO) 1.08 1.05 Example 5 Zroxide(Zr₂O) 1.08 1.05 Example 6 Fe oxide(FeO) 1.05 1.00 Example 7 Nboxide(Nb₂O₅) 1.09 1.05 Example 8 Ni oxide(NiO) 1.06 1.05 Example 9 Cooxide(CoO) 1.06 1.00 Example 10 Si oxide(SiO₂) 1.09 1.05 Example 11 Fe1.04 1.00 Example 12 Ni 1.03 1.05 Example 13 Co 1.04 1.00 Example 14 Cuoxide(CuO) 1.12 1.00 Example 15 Fe(Fe(BF₄)₂) 1.03 0.90 Example 16Ni(Ni(BF₄)₂) 1.00 0.95 Example 17 Co(Co(BF₄)₂) 1.02 0.90 ComparativeMn >1.5 1.00 Example 2 Comparative Mn oxide(MnO₂) >1.3 1.05 Example 3Comparative Mg sulfate(MgSO₄) >1.2 1.24 Example 4

As is apparent from Table 1, it is found that the secondary battery ofComparative Example 1 using a negative electrode active material, whichis not treated by surface-modification such as metal coating, is swelledand increased in change in the thickness of the battery when the batteryis stored at high temperatures.

It is understood that each secondary battery obtained in Examples 1 to13 using a negative electrode active material which is surface-modified,on the other hand, is reduced in the swelling of the battery and inchange in the thickness of the battery when it is stored at hightemperatures. It is understood that, in particular, the battery ofExample 1 used the negative electrode active material having a coatinglayer made of Mg oxide and the batteries of Examples 11 to 13 and 15 to17 used the negative electrode active materials having coating layersmade of Fe metal, Ni metal and Co metal respectively are further reducedin the swelling of the battery.

On the other hand, it is also found that the secondary batteries ofComparative Examples 2 and 3 using a negative electrode active materialshaving coating layers made of an Mn and an Mn oxide formed on itssurface, respectively, and the secondary battery of Comparative Example4 using a negative electrode active materials having a coating layermade of an Mg sulfate are increased in the swelling of the battery andalso in change in the thickness of the battery similarly to the batteryof Comparative Example 1 which is not surface-modified.

It is also found that favorable large-current characteristics areobtained by each secondary battery of Examples 1 to 17 from the factthat though the negative electrode active material of the negativeelectrode of each secondary battery of Examples 1 to 17 is modified by aspecific surface coating material, it exhibits the same batteryresistance as that of Comparative Example 1 provided with the negativeelectrode containing a negative electrode active material not formed acoating layer on its surface.

Moreover, the negative electrode of a non-aqueous electrolyte batteryassembled in the same manner as in Example 13 or Comparative Example 1was subjected to XPS analysis.

The non-aqueous electrolyte battery of Example 13 was decomposed in aninert atmosphere before the evaluation test to take out the negativeelectrode. The negative electrode taken out was cut into a desired sizeto make a sample, which was then subjected to XPS analysis under thefollowing condition while it was kept in an inert atmosphere.

1. Pretreatment

For all samples are measured under no contacting with an atmosphere,sampling each samples and transferring each samples to an instrumentwere carried out in an inert atmosphere.

2. Measuring Conditions

Instrument: Quantera SXM (manufactured by PHI)

Exciting X-ray: Monochromatic Al Kα_(1,2) ray (1486. 6 eV)

X-ray diameter: 200 μm

Photoelectron escape angle: 45° C. (inclination of a detector withrespect to the surface of the sample)

3. Data Processing

Smoothing: 9 points smoothing

Calibration of the abscissa: The main peak of C1s is set to 284.6 eV.

The obtained XPS chart is shown in FIG. 5. The peak derived from the2p_(3/2) binding energy of Co was detected at 776.6 eV from this XPSchart, which suggested that Co existing on the surface of the negativeelectrode was in a metal state. Also, as a result of the compositionanalysis using XPS, the existential amount of Co was 0.5 atomic %.

Also, the non-aqueous electrolyte battery of Example 13 was decomposedin an inert atmosphere after the above evaluation test to take out thenegative electrode. The negative electrode taken out was subjected toXPS analysis in the same condition as above. The obtained XPS chart isshown in FIG. 6. In the case of the negative electrode in the battery ofExample 13, the peak derived from the 2p_(3/2) binding energy of Co wasdetected at 776.6 eV also after the evaluation test like that detectedbefore the evaluation test from this XPS chart, which suggested that Coexisted in a metal state on the surface of the negative electrode.

On the other hand, the non-aqueous electrolyte battery of ComparativeExample 1 was decomposed in an inert atmosphere before the aboveevaluation test to take out the negative electrode. The negativeelectrode taken out was subjected to XPS analysis in the same conditionas above. Also, the non-aqueous electrolyte battery of ComparativeExample 1 was decomposed in an inert atmosphere after the aboveevaluation test to take out the negative electrode. The negativeelectrode taken out was subjected to XPS analysis in the same conditionas above. The obtained XPS charts of the negative electrode before andafter the test are shown in FIGS. 7 and 8, respectively. It is foundfrom these XPS charts that in the negative electrode in the battery ofComparative Example 1, the existence of Co is not confirmed eitherbefore or after the test.

Each of batteries obtained in Examples 15 to 17 degreases a change inthe thickness, and has more excellent large-current characteristicscompared with each of batteries obtained in Examples 11 to 13.

The battery of Example 15 was decomposed in an inert atmosphere beforeand after the above evaluation test to take out the negative electrodeand then was subjected to XPS analysis in the same condition as above.As result, it is confirmed that the peak derived from the 2p_(3/2)binding energy of Fe exists at about 707 eV either before or after theevaluation test. That is, it was found that Fe coating layer was formedin a metal state on the surface of the negative electrode. This iscaused by mechanism that Fe(BF₄)₂ added to liquid non-aqueouselectrolyte is ionized Fe²⁺ ions and BF₄ ⁻ ions, and then ionized Fe²⁺ions are precipitated Fe metal on the surface of the negative electrodeat an initial charge.

Also, the batteries of Examples 16 and 17 were subjected to XPS analysisin the same condition as above, respectively. As result, in the case ofExample 16, it is confirmed that the peak derived from the 2p_(3/2)binding energy of Ni exists at about 852 eV. In the case of Example 17,it is confirmed that the peak derived from the 2p_(3/2) binding energyof Co exists at about 777 eV. That is, the battery of Example 16 wasfound that Ni coating layer was formed in a metal state on the surfaceof the negative electrode in the same as the battry of Example 15. Thebattery of Example 17 was found that Co coating layer was formed in ametal state on the surface of the negative electrode.

According to the method of Examples 15 to 17, a thin metal coating layercan be uniformly formed on the negative electrode. Therefore, the methodcan be improved large-current characteristics compared with batteries ofExamples 11 to 13, which are used a negative electrode active materialpreviously formed a metal coating layer on its surface.

Embodiments of the present invention being thus described, it will beobvious that the present invention is not limited to the aboveembodiments and various modifications are therefore possible withoutdeparting from the spirit and scope of the present invention defined bythe appended claims. Also, the present invention may be varied withoutdeparting from the spirit of the invention when it is embodied. Further,various inventions may be formed by appropriately combining a pluralityof structural elements disclosed in the above embodiments.

1. An active material for a battery, comprising a lithium-titaniumcomposite oxide particle, and a coating layer formed on at least a partof the surface of the particle and including at least one metal selectedfrom the group consisting of Mg, Ca, Sr, Ba, Zr, Fe, Nb, Co, Ni, Cu andSi, an oxide of at least one metal selected from the group or an alloycontaining at least one metal selected from the group.
 2. The activematerial according to claim 1, wherein the metal is at least one metalselected from the group consisting of Mg, Fe, Ni and Co.
 3. The activematerial according to claim 1, wherein the coating layer has a thicknessof 1 to 100 nm.
 4. The active material according to claim 1, wherein themetal, the oxide of the metal, or the alloy is amorphous.
 5. The activematerial according to claim 1, wherein each of peaks derived from a2p_(3/2) binding energy of Co, Fe, Ni and Mg in photoelectronspectroscopy exists in the range of 775 to 780 eV, 704 to 709 eV, 850 to854 eV and 48 to 50 eV, respectively.
 6. The active material accordingto claim 1, wherein the amount of each of Co, Fe, Ni and Mg incomposition analysis using photoelectron spectroscopy is 0.1 to 1.0atomic %.
 7. The active material according to claim 1, wherein thelithium-titanium composite oxide has a spinel type structure or aramsdellite structure.
 8. A non-aqueous electrolyte battery comprising:an outer package; a positive electrode housed in the outer package; anegative electrode housed with a space from the positive electrode inthe outer package and including an active material; and a non-aqueouselectrolyte filled in the outer package, wherein the active materialcomprises a lithium-titanium composite oxide particle, and a coatinglayer formed on at least a part of the surface of the particle andincluding at least one metal selected from the group consisting of Mg,Ca, Sr, Ba, Zr, Fe, Nb, Co, Ni, Cu and Si, an oxide of at least onemetal selected from the group or an alloy containing at least one metalselected from the group.
 9. The battery according to claim 8, whereinthe metal is at least one metal selected from the group consisting ofMg, Fe, Ni and Co.
 10. The battery according to claim 8, wherein thecoating layer has a thickness of 1 to 100 nm.
 11. The battery accordingto claim 8, wherein the metal, the oxide of the metal, or the alloy isamorphous.
 12. The battery according to claim 8, wherein each of peaksderived from a 2p_(3/2) binding energy of Co, Fe, Ni and Mg of thenegative electrode in photoelectron spectroscopy exists in the range of775 to 780 eV, 704 to 709 eV, 850 to 854 eV and 48 to 50 eV,respectively.
 13. The battery according to claim 8, wherein the amountof each of Co, Fe, Ni and Mg in composition analysis of the negativeelectrode in photoelectron spectroscopy is 0.1 to 1.0 atomic %.
 14. Thebattery according to claim 8, wherein the lithium-titanium compositeoxide has a spinel type structure or a ramsdellite type structure. 15.The battery according to claim 8, wherein the positive electrodecontains a manganese-containing lithium-transition metal compositeoxide.
 16. The battery according to claim 15, wherein themanganese-containing lithium-transition metal composite oxide has aspinel type structure.
 17. The battery according to claim 8, wherein thenon-aqueous electrolyte contains a solvent obtained by mixing two ormore solvents selected from the group consisting of propylene carbonate,ethylene carbonate and γ-butyrolactone.
 18. A battery pack comprising aplurality of the non-aqueous electrolyte batteries according to claim 8which are electrically connected each other in series, in parallel, orboth in series and in parallel.
 19. The battery pack according to claim18, wherein further comprises a protective circuit which detects thevoltage of each of the batteries.